In modern cellular radio systems, the radio network has a strict control on the behavior of the terminal. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station to the terminal.
At power-on or after a long standby time, the User Equipment (UE) is not synchronized in the uplink. An uplink frequency and power estimate the UE can derive from the downlink (control) signals. However, a timing estimate is difficult to make since the round-trip propagation delay between the base station (Node B) and the UE is unknown. So even if UE uplink timing is synchronized to the downlink, it may arrive too late at the Node B receiver because of the propagation delays. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network. After the RA, Node B can estimate the timing misalignment of the UE uplink and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Random Access Channel (RACH) is provided for the UE to request access to the network. An Access Burst (AB) is used which contains a preamble with a specific sequence with good Auto-Correlation (AC) properties. The RACH can be orthogonal to the Traffic Channels (TCH). For example, in GSM a special RACH slot is defined. Because multiple UEs can request access at the same time, collisions may occur between requesting UEs. A contention resolution scheme has to be implemented to separate the UE transmissions. The RA scheme usually includes a random back off mechanism. The timing uncertainty is accounted for by extra guard time in the RACH slot. The power uncertainty is usually less of a problem as the RACH is orthogonal to the TCHs.
In WCDMA, the RACH is shared with the uplink TCHs. The uplink channels are not orthogonal. In addition to interference from other requesting UEs, the UE experiences interference from uplink TCHs and vice versa. The processing gain provided by the Direct-Sequence spreading will have to cope with the mutual interference. In WCDMA, the transmit power is a shared radio resource. In order to avoid near-far problems, the power received at Node B has to be approximately equal for each UE. A strict uplink power control is required. To prevent an AB to saturate the Node B receiver, the UE starts with a relatively low transmit power. If no access grant follows, it retransmits the AB at a slightly higher power level. It continues to increase the AB power until a grant is received.
To distinguish between different UEs performing RA typically many different preambles exist. A UE performing RA randomly picks a preamble out of a pool and transmits it. The preamble represents a random UE ID which can be used by the Node B when granting the UE access to the network. The Node B receiver can resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that multiple UEs simultaneously use the same preamble a collision occurs and most likely the RA attempts are not successful since the Node B cannot distinguish between the two users with a different random UE ID.
To minimize the probability of collision the set of available sequences should be large.
Preambles assigned to adjacent cells are typically different to ensure that a RA in one cell does not trigger any RA events in a neighboring cell.
As already stated before, the preamble shall possess good AC properties to guarantee a good detection (e.g. good timing resolution). To be able to reliably distinguish RA attempts performed with different preambles, good cross-correlation properties are important. In the ideal case, the preambles are orthogonal to each other (orthogonal means vanishing cross-correlation function over at least the time interval of possible roundtrip propagation delays). Many different preambles are desirable to lower the collision probability. However, the number of orthogonal preamble sequences is rather low. Therefore the two requirements—“many” and “orthogonal”—contradict each other.
The number of available orthogonal sequences is typically too low to fulfill the required collision probability—at least for the worst case assumptions on the RACH load. Therefore the sequences assigned to a cell are typically extended with non-orthogonal sequences. Since these additional preambles are not orthogonal, intra-cell interference occurs.
In case of high RACH loads this is unavoidable since additional sequences are required to lower the collision probability. However, for low RACH loads these additional sequences would not be necessary since the collision probability is anyway low—adding additional sequences only generates intra-cell interference.